The present invention relates to a film-coated cathode for alkali metal halate electrolysis, and to methods for preparing such cathodes. The cathodes of the present invention permit electrolytic operation with enhanced current efficiency without requiring the addition of chromate or dichromate ions to the halate solution.
Sodium chlorate is currently manufactured by the electrolysis of sodium chloride brine in electrolytic cells which provide for the mixing of the various electrolysis products: hydrogen, sodium hydroxide and chlorine. The electrolytic chlorate cells may be similar in design and configuration to chlor-alkali cells except that an intermediate barrier or cell separator is omitted to provide for product mixing. During the electrolysis of the chlorate solution, hydrogen gas is formed at the cathode along with sodium hydroxide while chlorine is formed at the anode. Hypochlorous acid and sodium hypochlorite are formed chemically in solution according to the following equilibrium reaction: ##STR1## Hypochlorous acid and sodium hypochlorite combine, under suitable conditions, to form sodium chlorate. The overall reaction can thus be described as follows: ##STR2##
Although the principal reaction at the cathode is the formation of hydrogen gas, there are other secondary reactions at the cathode which affect the efficiency of sodium chlorate production. Two such reactions are the cathodic reductions of sodium hypochlorite and sodium chlorate to sodium chloride and water as follows: ##STR3##
The reductions of hypochlorite and/or chlorate ions in the cell, as described by the foregoing reactions, are undesirable, inasmuch as said reductions can effectively result in a loss in current efficiency, as measured by the change in concentration of sodium chlorate in the solution, of as much as 80% or more.
It has been known for many years that the addition of small amounts of chromates, such as sodium chromate or sodium dichromate, to chlorate cell liquor, typically in amounts of 2 to 5 g./l. of solution, effectively prevents the reduction of hypochlorite and/or chlorate ions at the cathode, resulting in enhanced cell current efficiencies. It has been hypothesized that the effectiveness of chromates in chlorate production is due, at least in part, to the formation of a Cr(OH).sub.3 film on the surface of the cathode during operation of the chlorate cell in the presence of chromate, which prevents the cathodic reduction of hypochlorite and chlorate ions.
Unfortunately, however, the continued use of chromate additives in chlorate manufacture is subject to several limitations which have stimulated a search for more desirable substitutes. For example, world-wide supplies of chromium or are subject to disruption which could lead to limited supplies and/or higher costs for chromate additives. In addition, there are significant environmental restrictions associated with the use of chromates. Accordingly, there is a preceived and growing need for eliminating, or at least reducing, the use of chromate additives in chlorate cells.
It is thus a primary object of the present invention to provide a viable alternative to the use of chromates in chlorate cells, without sacrificing cell performance and efficiency. This object is achieved by providing a cathode having a protective porous film on the surface thereof.
In the past, it has been known to fabricate an electrode from components comprising a polymeric material, or to include a polymer or resin within the structure of the electrode. U.S. Pat. No. 4,118,294 to Pellegri, issued Oct. 3, 1978, discloses such a bipolar electrode for chlor-alkali electrolysis. The bipolar electrode comprises a valve metal anode having an anodically active surface bonded to a cathode formed from a powdered electrically conductive material embedded in electrically conducted relationship in a cured thermosetting resin, the surface of said cathode comprising a powdered catalyst material. Such bipolar electrodes are assembled by molding the cathode components to the valve metal anode structure and spraying the powdered catalytic material onto the cathode surface. The electrode of Pellegri functions to reduce the diffusion of atomic hydrogen into the electrode, but there is not recognition in the reference that such an electrode could serve to eliminate the use of chromate additives in chlorate cells. A similar disclosure is also found in U.S. Pat. No. Re. 29,419 to DeCraene, reissued Sept. 27, 1977, which describes an anode comprising a polymeric matrix containing catalyst particles bonded to a titanium substrate.
U.S. Pat. No. 4,101,395 to Motani et al., issued July 18, 1978, describes a cathode for chlor-alkali electrolysis having an ion exchange membrane laminated on its surface, the ionic exchange material comprising a polymer containing a cationic exchange group.
Although various attempts have been made to reduce or eliminate the use of chromates in sodium chlorate manufacture, including the addition of precipitants to the cell liquor or the use of ion exchange techniques, such attempts have heretofore not proven satisfactory from an economic standpoint.